- [Instructor] When you need to amplify a signal,or buffer a signal, filter a signal,or simply add two signals together,look no further than the operational amplifier,or op-amp for short.An operational amplifier is an electronic componentthat acts as a voltage amplifier,with an incredibly high amount of gain.It's DC coupled,which means it can be used to amplifyboth AC and DC signal components,it has two differential input terminals,and has a single-ended output.

Op-amps have become popular in modern electronicsbecause they're fairly cheap, easy to use,and they can implement a wide range of operationsfor processing electrical signals.If you look at the internal circuitryof an operational amplifier,you'll see a complex network of components.For example, the op-amp schematic shown herecontains 20 individual transistors, that are interconnectedto form three separate amplifier stages.The thing that makes op-amps so great,is that to use them,I don't need to understand how all of this complex,under-the-hood circuitry actually works.

I can treat that complex op-amp circuitas a single component,and I just need to remember a few simple rules to use it.The schematic for an operational amplifierlooks like a sideways pointing triangle,with five terminals.The two terminals on the left are differential inputs,the terminal on the top with the plus signis called the non-inverting, or positive terminal,and the terminal on the bottom with the minus signis the inverting, negative terminal.

As a differential amplifier, the input signal to the op-ampis the difference in voltage between those two terminals,subtracting the inverting input voltagefrom the non-inverting input voltage.The impedance of the input terminals on an op-ampis designed to be so high that, for most practical purposes,I can basically treat the op-ampas if it has an infinitely high input impedance.So, when I'm designing an op-amp circuit,I can pretend like no current will flowinto either input terminal.

That approximation keeps things simple,and is often referred toas one of the golden rules of op-amps.The output terminal on the right side is single-ended,meaning the output voltage will be referencedto the circuit's common ground.The output terminal is designed to have a very lowoutput impedance, so it can provide lots of currentto whatever load is connected to it,to produce the desired output voltage.The last two terminals on the top and bottomof the op-amp symbol,are connected to the positive and negative voltage linesthat provide power to the op-amp,which are called the power rails.

An op-amp cannot produce an output voltagethat is greater than the positive rail voltageor less than the negative rail voltage,so the range between those two supply voltagesneeds to be big enough to encompass the full rangeof the expected output signal.Different types of op-ampswill be able to handle different supply voltage rangesand will have their own requirements for the relationshipbetween the output signals they can produce,and the supply voltage levels.So, be sure to check the data sheet for that,when you're choosing which type of op-amp to usefor a certain circuit.

In schematics, it's common to hide the power railsto reduce clutter in the drawing,but, even if they're not shown,you always need to connect the power supply rails.Op-amps are an active componentthat require an external source of power to function.For prototyping circuits on a breadboard,I usually use op-amps that come in an 8-pin,plastic dual in-line package, or PDIP, form factor,which is a rectangular package with two parallel rowsof four pins.

Those pins are spaced apart just right,so that the component can straddle the dropthat runs down the center of the breadboardwith one row of pins on each side.I typically use a black wireto connect the negative supply rail to the op-amp,and a red wire to connect the positive supply rail.The pins on the PDIP are numberedcounterclockwise around the package,and the surface of the packagewill always have some sort of dot or orientation markingto indicate which side is which, so you can find pin #1.

Some op-amp models, like the 741 op-amp,only contain a simple amplifierwithin an 8-pin PDIP package.But other models, like the 358 op-amp,can have two separate amplifierspacked into a single package.The location of the input, output, and power supply pinswill vary for different types of op-amps,so always check the data sheetto make sure you're connecting the part correctly.It's easy to destroy an op-ampby accidentally connecting the wrong pin to power,and unfortunately, an op-amp usually doesn't showany visible signs that it's been destroyed.

To check whether an op-amp has been broken,you'll have to measure the output signalwith an oscilloscope, and decide if it lookslike it's supposed to.Since it functions as a differential amplifier,the output voltage from the op-ampwill be equal to the differenceof the non-inverting input voltage,minus the inverting input voltage,times a gain factor known as the open-loop gain,abbreviated here as AOL.Op-amps are designed to havean incredibly large open-loop gain,usually over 100,000, which makes them incredibly sensitiveto small differences between their input terminals.

For example, if the inverting input terminal was at 1 volt,and the non-inverting input terminal was at 1.001 volts,the difference between those two terminalsis just 1 millivolt, but, when the op-amp scales thatwith an open-loop gain of 100,000,it turns that tiny 1 millivolt differential inputinto 100 volt output signal.Now, as I mentioned earlier, the op-amp can't generateoutput voltages that are greater than, or less thanits power supply rail voltages, so,if I had my op-amp powered by plus and minus12 volt power sources,that 100 volt output signal would get clipped off.

At the very most, it can only be 12 volts,and depending on the actual capabilitiesof the op-amp I was using,it would probably be even slightly less than 12 volts.If I swap those two input signals,so that the inverting input voltagewas slightly higher than the non-inverting input,then the op-amp would see that difference as -1 millivolt,and its output would saturate in the other direction,at -12 volts.When the op-amp is used in this way,it's called the open-loop configuration.

The op-amp simply scales the inputby its enormous gain factor, and as you can see,it doesn't take much input to saturate the op-amp's output.That open-loop behavior can be useful for some applications,like using an op-amp as a comparator,which I'll cover later in this course,however, to keep those wild voltage swings under control,it's much more common to use a circuit configurationthat provides an external path from the op-amp's output,back to its input terminals,which creates a closed-loop configurationthat provides feedback.

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Released

9/8/2017

Bolster your understanding of how to build electronic circuits by learning to work with semiconductor components. In this course, join electrical engineer Barron Stone as he walks through how to build circuits using three common types of semiconductor components: diodes, transistors, and operational amplifiers. Barron kicks off the course by explaining what diodes are, and how to use diodes to control the direction that current flows through a circuit. He shows you how to use diodes to protect your circuits from large amplitude signals, reverse current, and flyback voltage. He then moves on to working with transistors, demonstrating how they can be used to control the amount of current flowing through a circuit, and examining common types of transistors such as BJTs and MOSFETs. Barron wraps up the course by covering one of the most useful electronic components—operational amplifiers. He shows you how to use op-amps to supply dual voltages, compare two signals, amplify signals, filter signals, and more.

Topics include:

Semiconductor materials

Diode applications

Rectifying a signal

Detecting the signal peak

Protecting against large signals, reverse current, and flyback voltage